The present invention relates to a digital copier capable of printing out an image with switchable pixel densities.
Generally, a digital copier has a reading section for reading an image to be copied and a writing section for writing the read image on a photoconductive element or similar image carrier. The writing section is often implemented by a laser printer. A current trend in the art is therefore toward the use of the writing section of such a digital copier as a printer of another machine such as a word processor or a facsimile apparatus and, in fact, many of modern digital copiers are so designed. A word processor, facsimile apparatus or similar machine produces outputs chiefly in the form of characters, as distinguished from pictures, and therefore does not need extremely high pixel density in printing. For this reason, a majority of printers currently used with this kind of machine has a pixel density of 240 dots per inch or 300 dots per inch, for example. The pixel density of a picture available with such a machine is also low. For example, a digital signal applicable to a word processor to print out characters has a pixel density which is as low as 24.times.24 dots, and therefore the pixel density of picture data available is also low. On the other hand, a digital copier needs and is usually provided with a pixel density of higher than 400 dots per inch so that it may print out images as faithfully as an analog copier. Assume that a character outputted by a word processor having a pixel density of 240 dots per inch, for example, is printed out by a printer which is installed in a writing section of a digital copier whose pixel density is 400 dots per inch. Then, the number of pixels per inch in the image signal from the word processor is 240 with respect to the main scanning line. However, since the number of pixels available with the printer is 400 per inch, the image is reduced to 240/400 when printed out. To compensate for such a reduction, a polygonal mirror forming a part of optics which is disposed in the writing section may have its rotation speed reduced to 240/400. Although this kind of implementation may guarantee 1:1 magnification, adjacent pixels will be spaced apart from each other in both of the main and subscanning directions because each pixel has a small diameter associated with the pixel density of 400 dots per inch, resulting in low image quality.